Optical properties of humic-like substances (HULIS) in biomass-burning aerosols

TLDR

The study presents optical properties of humic‑like substances isolated from biomass‑burning aerosol collected in the Amazon basin during the LBA‑SMOCC experiment in September 2002. Aerosol particles were generated from isolated HULIS, and their scattering, absorption, size distribution, and mass were measured to derive the complex refractive index via closure calculations. HULIS exhibited a complex refractive index of ~1.65–1.69–0.0019 i at 532 nm, with low mass absorption coefficients (~0.03 m² g⁻¹) that rise sharply at shorter wavelengths, reaching 2–3 m² g⁻¹ at 300 nm and contributing up to 50 % of aerosol absorption at that wavelength, while accounting for 6–9 % of total solar‑band absorption. Abstract.

Abstract

Abstract. We present here the optical properties of humic-like substances (HULIS) isolated from the fine fraction of biomass-burning aerosol collected in the Amazon basin during the LBA-SMOCC (Large scale Biosphere atmosphere experiment in Amazonia – SMOke aerosols, Clouds, rainfall and Climate) experiment in September 2002. From the isolated HULIS, aerosol particles were generated and their scattering and absorption coefficients measured. The size distribution and mass of the particles were also recorded. The value of the index of refraction was derived from "closure" calculations based on particle size, scattering and absorption measurements. On average, the complex index of refraction at 532 nm of HULIS collected during day and nighttime was 1.65–0.0019i and 1.69–0.0016i, respectively. In addition, the imaginary part of the complex index of refraction was calculated using the measured absorption coefficient of the bulk HULIS. The mass absorption coefficient of the HULIS at 532 nm was found to be quite low (0.031 and 0.029 m2 g−1 for the day and night samples, respectively). However, due to the high absorption Ångström exponent (6–7) of HULIS, the specific absorption increases substantially towards shorter wavelengths (~2–3 m2 g−1 at 300 nm), causing a relatively high (up to 50%) contribution to the light absorption of our Amazonian aerosol at 300 nm. For the relative contribution of HULIS to light absorption in the entire solar spectrum, lower values (6.4–8.6%) are obtained, but those are still not negligible.